Protective electric breakdown device



May 5, 1953 A, LQNGACRE 2,637,780

PROTECTIVE ELECTRIC BREAKDQWN DEVICE Filed May 6, 194; s. sheds-sheet 1 .DEVICE 5 VOLTAGE w SOURCE s v VOLTAGE Fl G- 6 SOURCE v l INVENTOR.

ANDREW LONG/xm;

, TTo RN'EY I.,

May 5, 1953 A. LONGACRE 2,637,780

PROTECTIVE ELECTRIC BREAKDCWN DEVICE Filed May e, 1945 s sheets-sheet 2 l/L TAGE Jol/RCE ro ANTENNA m R50e/vsn T0 TRANSMITFER /NVEN Tof? A.z o/v ACRE ORA/EY May 5, 1953` A. LoNGAcRE 2,637,780

PROTECTIVE ELECTRIC BREAKDOWN DEVICE Filed May 6, 1943 3 Sheets-Sheet 5 N VISA/ro@ A. LON CRE Patented May 5, 1953 PROTECTIVE ELECTRIC BREAKDOWN DEVICE Andrew Longacre, Belmont, Mass., assigner, by inesne assignments, to the United States of America as represented by the Secretary of the Navy Application May 6, 1943, Serial No. 485,933

This invention relates to switches of the automatic breakdown type for association with an electrical transmission line or wave guide for the purpose of protecting a sensitive apparatus connected to said transmission line. More particularly, this invention relates to automatic breakdown switches adapted for use in wave guides having the form of hollow pipes.

Because of the modern emphasis on phenomena in the space surrounding a transmission line as contrasted with the phenomena within the conductor, the term wave guide is now frequently used to refer to a transmission line, especially one used at high frequencies. This term is frequently applied to transmission devices in the form of a hollow pipe, which are practical only for high frequency applications. Properly speaking, any transmission line is a wave guide. Modiiying phrases are therefore liberally used in this specification to make it clear that the type of wave guide in conjunction with which the breakdown switch of this invention is particularly concerned is the form of wave guide constructed as a hollow pipe. In conformance with the terminology of the high frequency art, the term wave guide will generally be used herein in place of the term transmission line.

When intermittent trains of oscillations are being transmitted from a source of oscillation of relatively high amplitude, such as a generator of high frequency electromagnetic oscillations, to a terminal apparatus, such as an antenna system, over a wave guide and it is desired to connect with said wave guide a sensitive device such as a receiver for such high-frequency oscillations, it is desirable to protect the receiver against damage from oscillations of great amplitude by providing a switch or a protective gap between said receiver and said wave guide which will short circuit oscillations exceeding a predetermined amplitude across the receiver input. It is an object of this invention to provide a protective breakdown discharge device which will occupy verylittle space when associated with a hollow pipe wave guide system and which is simple in construction. It is a further object of this invention to provide such a device for use in hollow pipe wave guides which will take advantage of the symmetrical nature of hollow pipe wave guides and will take advantage of the symmetrical and essentially two-dimensional nature of resonant circuit constituted by an elongated aperture in a sheet of metal.

In the annexed drawings:

Fig. l is a diagram showing the location of the 25 Claims. (Cl. 178-44) protective breakdown discharge device of this invention in a radio system employing hollow pipe wave guides;

Fig. 2 is a perspective view of one form of the protective breakdown device of this invention together with associated portions of hollow pipe wave guides;

Fig. 3 is a perspective view showing another form of protective breakdown device according to this invention, and also showing associated portions of wave guides;

Fig. 4 is a perspective view of still another form of protective breakdown device according to this invention together with associated portions 'of wave guides;

Fig. 5 is a cross section showing a modied form of protective breakdown device;

Figs. 6 and 7 show, in cross section and in elevation, respectively, a modification of the protective breakdown device according to this invention;

Fig. 8 is a perspective view of a compound form of protective breakdown device in which two cooperating breakdown elements are provided at different parts of the hollow pipe wave guide system;

Fig. 9 is a median horizontal cross section of the apparatus shown in Fig. 8;

Fig. 10 shows in cross section another arrangement of the protective breakdown device of this invention in a wave guide system associated therewith;

Fig. 11 shows in cross section still another arrangement of the 'protective breakdown device of this invention in a wave guide system associated therewith;

Fig. 12 shows in cross section an arrangement for adjusting the resonant frequency of the protective breakdown device-that is, for tuning the device;

Fig. 13 shows, in elevation, an alternative arrangement for tuning thev protective device, and

Figs. 14 and 15 show, in elevation and in lplan respectively, still another arrangement for tuning the protective breakdown device. v

In the protective breakdown device of this invention, there is used as a circuit adapted to promote breakdown when subjected to oscillations exceeding a predetermined amplitude,l a sheet of conducting metal transversely closing the hollow pipe wave guide leading to the sensitive device (as shown in Fig. 1) and provided with an aperture of elongated contour so dimensioned and shaped that the metal sheet, in the neighborhood of said aperture, constitutes a respendicularly to the direction in which currents` tend to flow in the wave guide walls, so that the' resonant circuit will be excited by oscillations inv i the guide and a high VoltagegradientV will ap'- pear across the middle part of said. aperture. The aperture is constructed with a very narrow clearance in its middle portion so that when the oscillations reach a predetermined amplitude, a breakdown will occur across the narrow` portion of said aperture. rEhe resulting short circuit will in :effect electrically-close ofi the :hollowpipewave guide leading to. the sensitive deviceor receiver. At the same time, if the aperturedl metal partitionfisfprop'erly'locatedin' the hollow pipe wave guide leadingy to the: receiver,- as` hereinafterf described, the wave guide leading from. the source of oscillation to the terminal apparatus (see Fig. 11). will; when breakdownfoccursas aforesaid; act electrically' like anl essentially uninterrupted Wave guide'.

Fig. l. shows the generaiftypeiof system infwhich the breakdown switch ci this invention finds' its particular utility and` shows the {.eneral` location of' suchv` a breakdown svntch in such a System@ The block I indicates in a general. mannen' a source of highlamplitude, which could be a vacuum tube oscillator; The-wave guide 2 is atx-ansmission apparatus of the hollow pipe-type which ispreferablyrectangular incross section (though the invention'isequally applicable tosystems with wave guides of circular cross section) and of such dimensionsthat it readily sustainsfandtransmits oscillations' of theV frequencyv of those generated in the source l winchv are'so polarized in` said waveguidethat the electric'vector of suchV oscill lations lies in the direction of the short dimension of said wave guide, but does not. readily' sustain or transmit other modes of oscillation at seid frequency; The'wavefguide. 21 serves to transmit oscillation-strom the' source. I: to a termina-l appa- I ratus generally indicated a't 3; which may bean antenna system. Intermediate the ends ofA the guidez' atk al suitable point determined'by methods known tothose-'skilled in the-art, another hollow pipe'wave'guide e is joined to the guide'v 2f. The i guide 4 likewise functions to transmit oscillations of which` theelectric vector is parallel toits shorter cross-sectional dimension". The guide 4 serves to lead or transmit oscillationstaking place in the guide 2 to a sensitive device generaily'indicatedat' 5', whichmay be a receiver for detecting and amplifyinghigh-frequency signals. In the guide:` 4,. in this:A case at its extremity' where: it joins the. guide' E, there is placed` a transverse barrier 6 of a thin conducting material, prefer.- r.:

so that breakdown will readily occur when oscillations of such amplitude as might damage the sensitive device 5 occur in the guide 2. Oscillations of such amplitude might, for instance, occur whenever the source of oscillation l is delivering energy to the guide 2, this source is a source oft' relatively high-amplitude oscillations. When oscillations of such amplitude are plSf-mt in the guide 2 and a breakdown occurs across the narrow portion of the aperture in the barrier 6, the guide 2 electrically acts as ii it were completely cut OIT from the pipe Il by a continuous conducting barrier, except for the relatively low and;y practically negligible resistance associated with the breakdown discharge. When oscillations of suiiicient` amplitude to cause a breakdown across the. aperture are not present, however, the aperture will readily permit the transmission therethrough of oscillations of frequencies in the neighborhood of the frequency at which the circuit surrounding the aperture resonates'sothat the sensitive devicev 5 is effectively connected' with thete'rniinal apparatus 3 for receiving electromagnetic oscillations ot such frequencies.

In a system suchy as thatv of Fig; l itmay be desirable tofvary thelength of they guide 2` between the transmitter and the junction with the guide 4 in order to obtain an adjustment that minimizes absorption of received signals in the transmitter and itsv associated guide. Such ad justment may be readily determined experimentally. Absorption'oi. received signal'by the transmitter may also be prevented by other means, such as by a resonant side stub which actsl as a choke whilethe apparatus is used for receiving and whichis-adaptedtobe` short circuitedout of thefsystem by a gap discharge occurringwhile the apparatus is used for transmission.

Fig.,2 showsaperspective view of the junction of the-guides 2f and liy of Fig. l, thus illustrating in'detailthe 'form-of one embodimentvofthis invention. The conducting barrier 8 which closes off the end of: the guide tis shown as constituting a portion o-f the wall ofthe guide These guides, 2f anda, are manufactured in the form or thinwalled rectangular metallic pipe, preferably of Copper or brass.V The aperture inthe barrier 6 is shownat. Although for the purposes of this invention any of` avariety of shapes, usually elongated in some mannen will serve for this aperturelprefer theshape shown in. Fig. 2 whichvS essentiallyy that of a long narrow slit terminated at its. end by rounded openings of a diameter somewhat wider than .the saidslit.

When oscillations exceeding. a predetermined amplitude occur in the guide 2, a breakdown occurs across thejaws Il of the slit forming a part ofy aperture 8.

Fig. 3 shows an. alternativer arrangement. of the protective. breakdown discharge device of thisl invention. In Fig. 3l the junction between they wave, guides 4 and 2 is made in one of the narrow sidesfof the waveguide 2 instead of in. one ofthe broader sides as in. Fig. 2. The junction shown in, Fig. 2 is known as. an electric plane junction while the junction shown. in Fig. 31S known asainagneticplane junction. Ingeneral. the type of junction sl'iownV inV Fig. 2. is at present believed tobe preferable.

It willbe noted thatinthe apparatus ofFig. 3 the aperture S is oriented lengthwise of the wave guide 2, although in they apparatus of Fig..2.the aperture 8 is oriented perpendicular tothe axisof the wave guide 2. In each case, however, the

aperture 8 is oriented so that potential differences will appear across its narrow dimension. `In both Fig. 2 and Fig. 3 the aperture 8 has the same orientation with respect to the branch wave guide 4. It is oriented at right angles to the direction of the electric vector in the branch Wave guide 4, the electric vector in a rectangular wave guide aS normally used being perpendicular to the broad sides of the wave guide and parallel to the narrow sides.

Fig. 4 shows a modified arrangement of the invention employing an improved type of protective breakdown aperture. In Fig. 4 the conducting barrier 6 is not, as in Figs. 1, 2 and 3, ush with the Wall of the wave guide 2 so as to form a continuation thereof, but is instead at a position farther down the guide in the direction of the sensitive device, an arrangement which I prefer in practice to that of Fig. 2. The distance between the transverse barrier 6 and the nearer wall of the wave guide 2 should be approximately equal to an integral number of half-wave lengths of the oscillation in the guide 4. A single halfwave length is preferred because greater lengths introduce undesired amounts of frequency sensitivity. In practice the distance will be somewhat less than that just defined, presumably on acount of the end effect associated with the junction of the two wave guides and the electrical characteristics of the breakdown discharge.

This distance is shown on Fig. 4 by the dimension a, and in the case of the electric plane junction shown in Fig. 4 its preferred vmagnitude depends to some extent on the ratio b/xg, in the manner illustrated in Table I, b being the narb/M am o. 22 0. 463 o. 25 o. 45s

The wave length of the oscillations in the guide is ordinarily diierent from the wave lengths of oscillations of the same frequency in free space,

and is in the usual case considerably longer than i the free space wave lengths. The wave length of oscillations of a given frequency in a wave guide of given dimensions may be calculated Without diiculty from the guide dimension by known methods or may be determined experi.- mentally by known techniques.

The barrier B and its aperture 8 as shown in Fig. 4 are provided with means comprising aglass envelope I2 for maintaining a partial vacuum in the aperture 8 and thereby promoting electrical breakdown discharges in said aperture at voltages lower than those which would be necessary to produce breakdown across the saine aperture filled with air and atmospheric pressure. In this manner a greater degree of protection can be furnished to the sensitive device because protective breakdown will take place at a lower amplitude level.

The residual gas inside the envelope l2 may be air, or it may be composed of a gas or a mixture of gases adapted to be ionized more readily than air by the alternating electric field set up across the narrow part of the aperture 8. Water vapor or hydrogen might, for instance, be used. Neon and argon have slow de-ionization time characteristics and are therefore suitable only 'where relatively slow recovery (several microseconds or so) can be tolerated, as in the case of long-range radio-echo locating equipment.

Various degrees of partial vacuum might be used, depending on the desired amplitude level of oscillations at which it is intended the breakdown should begin to take place. A suitable partial vacuum for a sensitive gap, for instance, is one in which the pressure corresponds to a '7 mm. column of mercury. In general I prefer pressures between l mm. and 25 cm. of mercury. If long service life is more important than the ultimate in sensitivity, a relatively high pressure gap should be used which, if properly constructed, will provide'satisfactory sensitivity for all practical purposes. In the construction of such a gap great care should be taken to clean thoroughly the metal surfaces in the neighborhood of the discharge gap. If then an inert gas, such as nitrogen, is used for the'gap atmosphere, the discharge will be in the nature of a gas discharge rather than a metallic arc, and the sensitivity of the gap will be great even at relatively high pressures, such as 10 cm. of mercury, which I believe to be preferable. This discharge, because of the relatively high pressure will tend to remain conned to the space dened by the discharge aperture, even in apparatus operating at high power levels, which is a great advantage, since spread-out discharges may interfere with power transfer between transmitter and antenna. The service life of the type of discharge element just described is very great because the metal parts are y practically unaffected by the type of dicharge obtained.

One way in which the means for maintaining partial vacuum may conveniently be associated with the conducting metal wall 6 is more clearly illustrated in the cross-sectional View shown in Fig. 5 of the metal wall 6 of Fig. 4 and its associated glass structure. The glass envelope l2 is preferably formed of two parts, one on each side of the metal sheet 6. Each part is sealed to the metal wall 6 by a metal-to-glass seal about the circumference of the envelope, surrounding the aperture 8. The envelope l2 extends slightly away from the metal sheet 6 in its middle portion in order that the breakdown discharge shall not be interfered with.

In order to promote the breakdown discharge by establishing a partial vacuum in the aperture of the barrier 6, it is of course not necessary that a partial Vacuum should be maintained in all parts of the aperture and a glass envelope could be arranged to maintain a partial vacuum only in the narrow part of the aperture. This, however, would put part of the glass wall directly across the narrow part of the slit, which would tend to cause dielectric losses, so that such a form of the protective discharge device is not preferred.

Figs. 6 and 7 show a further modified and preferred form of the protective breakdown device of this invention. Fig. 6 is a sectional View in the median electric plane of the wave guide d. Fig. 7 is an elevation transverse to the axis of the wave guide 4.

In Figs. Gand 7, as in Figs; 4 and 5, the part of the barrier 6 in which the aperture 8 is cut is through the seal I5.

fonia/'7,1780

provided withfa glasslenvelope offtwo parts If2. I2 which maintaimafpartialyacuumin the 'aperture and yin thespacefneighboring the aperture. .In

addition, .passage I3 is idrilled :inside 4the 'metal passage i3 is providedanzelectrode Miwhichis .insulated from fthe -metal sheet :6 by a 'spacing 'bead Ma on the lelectrode .14. extends almost .totheipoint lwhere .thezpassage I3 The :electrode i4 reaches the apertures, but it is'terminatedzwithin the passage i3. ,A glass `seal `is lprovided :at vthe end of the :passage I3 which communicates 1with. the outside of :the waveguidein which-the sheet (i vis located, Athe electrode I4 :passing The1seal I5 `serves vto cooperate with the ,envelope ft2, I2 to 4rnaintaina 4partialvacuum in the space defined :bythe laperture. An electric potential, preferably a constant voltage, is impressed-upon the electrode :i4

`with respect to the waveguide 4 and thesheetf y(which are electrically connected) .from a source vof voltage schematically indicated by `block I9. lPreferably, a high .voltage is impressed :upon the electrode le through a high resistance, so that vwhen a-discharge begins to talreplace, the voltage will automatically be reduced. .For example, a

:voltageof 100) volts may be impressedupon the electrode I 4 through a high resistance of approximately 5 or k1() megohms.

The electric potentialibetween the electrode i4 and the walls-of the passageltwillcause a small amount offioniza-tionto take 4place in therpassage 5.

I3 which, like the space 1in .the aperture 8, Jis maintained under partial vvacuum conditions. Certain of the ions 'formed-in -this manner vwill ,migrate into the apertures, -so that when ahigh voltage appears across 1theaperture 8, a .breakdown will readily occur. In this fashion the `voltage necessary-tocause a breakdown is made considerably lower `than `if there were no `ions present in the aperture .8.

When the vapparatus of Figs. V6 and .7 is used in a system .similar to thatof Fig. l and in which .the source of oscillations is Aa'generator of high .frequency oscillations which transmits such oscillations in theform of kshort pulses'or otherinter- ,mittentvwave trains of highamplitude .of 4oscillation, the potential applied on the electrode I4 .in the apparatus of Figs. 6 and 7, insteadfoi-being a continuous constant potential, might be an intermittently. applied potential applied only during the period during which thesaid source of oscillations is transmitting energy. .Such agpotential may readily be applied by locating-asynchronized rapidly-operating electronic switch 'in the circuit of voltage source I9 and thefelectrode Hi, the ,switch being operated in a well-known manner by circuits associated with the source of oscillation and the apparatus controlling it. Constant potential operation of the electrode-is, however, preferred.

Apparatus maintaining a partial vacuum .for the discharge spacehas the disadvantage at high power operation that the discharge rtends to broaden out from the plane ofthe aperture 8 and to iill the rest of the space in which a partial vacuum is maintained. This operates to make the 4location of the short-.circuited termination in the wave guide i indeiinite and .makes.it-didin cult to obtain the desired reduction otenergyacceptance bythe wave guide .4. Inorder toavoid this diiculty, apparatus `may be .built shown Cil :but which is not shown in the figure.

ffii in Fig. 18, with vaflrst discharge 'aperture operating at higher pressure which is adapted to keep its discharge substantially coniined to the plane `in question while permittingidesirable conditions lto Aholdat thewave guide junction.

Fig. 8 is a perspective View and Fig. 9 is an felectricplane section of one lform of the compound ytype yof device just vreferred to. In this arrangement there areilocated in the guide `ll two conducting transverse barriers '6 and I6. The 'barriervli has an aperture 8 and is arranged and located exactly vas in Fig. 2. The barrier l5 is ,located in .the guide 4 between the barrier 6 and the sensitive device to which the guide fl leads The distance along the guide 4 between the barriers .5 and I6, indicated by the dimension h on Fig. 9, .should be approximately equal to an Yod-:lnumber .of electrical quarter-wave lengths inthe guide 4 of the'oscillations to which the system is designed to respond. For purposes of simplicity and economy of space I prefer-a spacing of one quarterwave length-between the 'barriers .6 and ifi. ,The barrier L6 is provided with :an aperture i8, an envelope I2 and an electrode I4, corresponding .fully yto the aperture 8, the envelope t2 and the electrode I4 of ligsl and '7. As in the latter `figures, Aa partial vacuum is maintained vin the immediate neighborhood ofthe aperture i8. It is to be notedfthat a great dealoi the total breek- -downenergy in the compound device isconsumed in the aperture 8 which is more resistant to damage and more readily and economically replaceable than the more sensitive element (the-barrier I6 and aperture I8) of `the device.

It is not necessary for this invention that the wave guide T-junction should be sc arranged that the branch leg vof the yT leads towards the receiver while the cross-arm leads in one direction to the antenna .and in the other direction to the transmitter yalthough that configuration is preferred. This preference is based on observations indicating that the most complete energy transfer between two legs of a T-junction of which the third leg has been short-circuited is `obtained when the short-circuited leg is the branch guide rather than part of the cross-arm lof the T. Consequently a straight path past the lT-junction is preferably established for the transmitting operations (corresponding to the existence of breakdown in the aperture In some cases, however, it may be found advantageous to provide a straight-through path for receiving, permitting transmission to take place around the wave guide corner of the T-junction. YArrangements of this type are shown in Figs. lo and '11, the connections to the transmitter and receiver being indicated on the iigures.

In Fig. 10 is shown theparts of a wave guide system which are located near the junction of the waveguides leading to the antenna, receiver and transmitter. The wave guides shown are, as before, in the 'form of rectangular vhollow conducting pipes. The wave guide 20 leads toward theantenna, the waveguide 2| leads toward a receiver .and the wave .guide 22 leads toward .a

transmitter. In the wave guide 2l is interposed an electrical breakdown device similar to that shown in Figs. 4 and 5, and comprises a metallic barrier 5, an elongated aperture 8, and a glass envelope i2 for maintaining a partial vacuum in the aperture 8. As in previous instances, the glass envelope l2 may be omitted if desired. The distance between the barrier 6 and the T-junction of the wave guides 20, 2| and 22 should, in accordance with the principles previously outlined, be approximately an integral number of electrical half-wave lengths. Although the effective location of the junction with respect to the branch wave guide of a T-junction, as, for instance in Fig. 4, lies approximately at the nearer wall of the cross-arm wave guide of the said T- junction, the effective location of the junction with respect to apparatus located in one of the cross-arm wave guides is in the neighborhood of the median plane of the branch wave guide, or, otherwise stated, the end effects are different in the leg and in the cross-arm of a T-junction. The desired dimension c indicated on Fig. 10 is slightly more than half the wave length of oscillations in the guide on account of the end effects at the junction. For a guide in which b/ \g=0.22, a preferred value for the dimension c is 0.52 times the wave length in the guide. Even with the preferred value of the dimension c a mismatch will occur at the junction unless further measures are taken, on account of the reel (conductance) component of the junction admittance as seen from the transmitter branch. For maximum energy transfer upon transmission, therefore, a matching device is necessary in the guide 22, which may be a pair of capacitive loads located about a quarter-wave length in the guide from each other.

Fig. 11 shows an arrangement similarto Fig. 10, except that the protective electrical breakdown device is lo-cated at the T-junction instead of some distance down the wave guide 2| away from the T-junction. In such a case the barrier 6, as shown in Fig. 11, is preferably so located that when a breakdown occurs in the aperture 8 a conducting wall electrically substantially continuous appears across the wave guide 2l in such a position as is most conductive for transmission of energy from the wave guide 22 around a right angle corner and down the wave guide 20. In view of this consideration I prefer a diagonal arrangement of the barrier 6 as shown in Fig. 11. The diagonal part of the barrier '6 is very slightly removed from the central diagonal of the T- junction in the direction which permits a slightly greater clearance around the corner of the wave guides 22 and 2|, in accordance with the practice for making right angle bends in rectangular wave guides. For an electric plane junction, such as that shown in Fig. 1l, I prefer arrangements in which the dimensions shown at d on Fig. 11 lie between 85 and 90% of the cross section of the wave guide in the direction of the electric vector. For a magnetic plane junction I prefer a slightly wider corner, with a dimension corresponding to the dimension d, of approximately 95% of the cross section of the wave guide in the magnetic plane. These preferences apply particularly to rectangular wave guides for which b/M as heretofore deiined is equal to about 0.25. j

For the best operation of the breakdown switch of this invention, it is important that the aper-` ture 8 should be rather accurately tuned to the frequency of the oscillations which are desired to be handled in the guides 2 and 4. Means for adjusting'` this tuning of the aperture 8 without removing the barrier 5 from the inside of the pipe system are therefore desirable, both for adjusting the breakdown switch to the particular frequency of oscillations transmitted, and for correcting any change in the dimension of the aperture 8 caused by the subjection of the narrow part of the aperture to repeated electric breakdown discharges.

A few of the many possible methods for accomplishing such tuning are shown in Figs. 12 to 15 inclusive. In Fig. 12 a threaded stud 30 is shown fastened to one of the walls of the wave guide The surface of the wall to which the stud t@ is fastened is perpendicular to the electric vector of the oscillations in the pipe. It is preferable to place the threaded stud 30 centrally with respect to the wave guide wall in order to locate it at or near the maximum electric field. The wall of the wave guide is perforated so that a screw 32 may be inserted into the pipe through the stud 32. A locking nut Sil serves to preserve the adjustments and to maintain drm electrical contact at the screw threads. Instead of providing actual contact at the wave guide wall between the wall and the screw 32, other arrangements providing effective energy transfer at radio frequencies by the use of suitable small resonators around the screw, with or without direct electrical contact, may be employed.

The distance between the screw 32 and the barrier E has an important influence upon the effect of the screw 32 upon the resonant structure of the barrier 6 and its aperture 8. If this distance is about one-half of the wave length in the wave guide of the oscillations in question, the eect of the screw will be that of an adjustable capacitance in parallel across the jaws of the aperture 8. If the distance between the screw 32 and the barrier Si is approximately one-quarter of a wave length of the oscillations in question, in the wave guide the adjustable screw, as it affects the aperture 8, will act as a Variable inductance across the width of the aperture 3. The eifect of the screw 32 when placed at positions intermediate of those just mentioned is somewhat more complicated, and consequently I prefer either quarter-wave or half -wave spacing or both.

A `similar screw introduced into the guide in a direction perpendicular to the electric vector of transmitted oscillations might also be used for tuning. Such a screw usually has an inductance effect at the place of its insertion. Another possible tuning arrangement is a branch or stub guide having a short-circuiting termination which is adjustable in position. These various tuning arrangements may be located at 'some distance from the barrier B, preferably one or more quarter-wave lengths, and yet eifectively provide an adjustable susceptance in shunt with the tuned circuit constituted by the barrier 6 and its aperture 8.

Fig. 13 shows another method for varying the tuning of the aperture 8. In this case, as shown in the drawing, the broadened ends of the aperture are made to occupy a somewhat greater part of the length of the aperture than in the aperture previously described, such as that shown in Fig. 2. The jaws 9 bordering the narrow part of the aperture may advantageously be formed of pieces or slugs 4l of a refractory metal such as tungsten (as shown in Fig. 13) in good electrical and physical contact with the highly conducting metal of the barrier 6, which is prefaamso the narrow portion of. the. aperture.. in. the region Where breakdown discharges occur. Since tungsten or other similar refractory metalwill withstand' the high temperatures. present4 in an arc discharge without.. Corrosion. 0r. disintegration, the facing, greatly lengthen-s theY useful life of the barrier. In one ofthe broadened ends of' the aperture 8v isprovided a swinging Vane 4D. A channelA is out in the metal of the barrier 6. in a direction parallelto, the long` dimensionof the aperture 8 and preferably located'centrally with respect to the short dimension thereof..` In. the channel is laid a shaft 4'4, which is fastened to the vane 40 and' serves to rotate the vane 4l! in the aperture 8i. The shaftA 44 is provided with a manipulating and. indicatingmeans, such as the crank E; in any well-knownmanner, Other mechanical arrangement-,could also be employed for actuating a vane similar to the` vane 4D. in the aperture 8,

Figs. l4jand 15j show, in elevation and planv respectively, s tillanother way of` tuning the aperture 8. In this arrangement,A sliding shutters 50, 5B made of metallic conductngmaterialmay be moved so as to` short-circuitthe-ends. of, the aperture 8, at an adjustable.-lo.c,ation The slideing shutters 50,50' are guidedby the lugs. 52, 52, 52, 52 which arev suitably fastened-tov the barrier 6, as by soldering. Theends of the shutters 50, 50' are brought, outside of the waveguide through suitable holes in. the Wave. guidewall. It is important thatthc shutters Ell, 50 should.- rnake good' electrical contact, with the barrier 6, and for thatllurpose spring, pressure may. be. applied by suitable means (not shown),

t is believedto be. desirabletoA keep-the, thicl ness of the metal barrier 6- downto fairly small proportions, in order. to minimize losses. The dimensions ofthe` aperture 8 suitable for opera.- tion at any particular'v frequency. will, of course depend upon the.vr shape ofthe aperture. For. a. given shape the desired., dimensions may, readily be determined by experimentalv procedures. In` general, it may bepointed. out thatv narrowingthe aperture in the directionof .the electricvector in the wave.g uideacross,.which the barrier 6 is locatedvhas aneiect. similar toanincrease of capacitancein a. tunedcircuit, while shorts., ening the aperture in, the` .direction perpendicular to the saidelectric vector. has-.aneiect sirnieA o tially two-dimensional system, and,I moreover, a..

symmetrical one., so. that a, ofmechanical construction is necessary. to arrange such an electrical breakdown. discharge device in a wave guide system. The, two-dimensional resonant aperture` has, in corrlparison, withother` resonant cavities that might be used for electrical.

breakdown switches in wave guide-systems, the

advantage of mechanical compactnessgandthe .further advantage of eXtreme-simplicityin-:theg

method of connecting the resonant.V apertureelectrically insuch a way as. t0.,respond to.. the. oscillations transmitted bythe wave guidesystem.

What I desire to claim andsecurebyA Letters. Patent is:

1,.. In a. system for. the transfer of high. fre-A quency electromagnetic waves including a pipe Wave-.guide,.a.protective electrical breakdown device, which includes a.. metallic conducting barrier located transversely of said wave gudeand having. an. aperture of elongated contour said. aperture being so dimensioned thatsaid barrier constitutes a tunedcircuit adapted to resonate ata frequency substantially equal to that of-oscillations desiredv to, ber transmitted through said wave guide system, said aperture being at least for` part ofv itslengthso narrow that when oscillations in, said pipe-exceeda predetermined am,- plitude an electrical breakdown discharge will occur across said. aperture.

2. In4 a system for transmitting and receivingby. means of.- the same antenna. and which in.- cludes pipe Wave guides adapted to transfer electromagnetic waves and leading respectivelytoward atransmitter, a receiver and an antenna,

the .combination of. a. suitably located junctionof` said wave guides, abarrler of conducting metal in that` oneof said wave guides-leading from .saidI junction to.` said receiver, said barrier having; an aperture of elongated contour. with. its` long, dimension oriented perpendicularly to-theelec.-

tric. vector.. of oscillations transmitted. in said. wave guide, the width of said aperturebeingsuf-r iicientlyJ small, atleast for. a part of its-length,

, t0 permit. the establishment of. an electrical.l breakdown. discharge across saidaperture.: when` oscillations irl-said wave guide exceed a prede termined. amplitude, said aperture being, dimenfsonedand shaped to. resonate at a frequencysubstantially. equal to that employed for said-.- transmitting andreceiving, said apertured.y bar rier being.Y located at adistancefrornsaid4 juno-h tion approximately equal to an integral, number- (includinazero) olhalfwave lengths-ofthe oscillations iii-said. wave guides..

3. Inia. system. for the transfer of. high fre-- Quency electromagnetic: wavesincluding apipewave. guide, a protective electrical breakdowndef vice. which includes a-metallic. conductingv barrier of. relatively thin. cross. section. located trans.-l

verselyof said wave guide and-having.. anfaper.-

ture ofelongated contour with its, longr .dimensioni perpendicular to. the electricvector. of oscillations. transmittedinsaid Wave guide. the widthofsacl-v aperture being sufliciently small at least-.for apartv ofits; length to. permit the-occurrence of arr-elec;-

tric breakdown. dischargey across said. aperture;

whenoscillations in said pipe exceed;a.predeterminedl amplitude, said4 aperture` being dimeri-v sioned. and Jshaped so` l'that vsaidapertured `barrier resonates. at afrequency. substantially equal. tothat of oscillations. transmitted` through said., wave.. guide system, and means associatedv with said, barrier` and said apertureto maintain apartial` vacuumin the vspace defined by said aperture for` promotingl ionization, and electrical.

breakdown. therein.-

relatively. thin cross` sectionvr located transversely of said. wave guide-- having an aperture, of elon-` gated .contour witlfi.A its. long dimension perbenedicular to the electric. vector'of. oscillation transmitted in said .wave guide, .the width of-,said-aperturc vbeing suiciently. small. at least forfapart.

of lits Alength to permit .the ,occurrence :of an elec.-

tric.` breakdown discharge across.y said aperture. whenoscillations in said pipe exceed.azpredeter..

mined amplitude, said aperture being dimensioned and shaped so that said apertured barrier resonates at a frequency substantially equal to that of oscillations transmitted through said wave guide system, means associated with said barrier to maintain a partial vacuum in the space defined by said aperture, an auxiliary electrode associated with said means and electrically separated from said conducting barrier for maintaining a relatively low intensity electric discharge in the immediate neighborhood of said aperture, and means for impressing a voltage on said auxiliary electrode with respect to said conducting barrier during periods when electrical ybreak-- down of said aperture is desired to be promoted.

5. In a system for the transfer of high frequency electromagnetic waves including a pipe wave guide, a protective electrical discharge device including a metallic conducting barrier of relatively thin cross section located transversely of said wave guide and having an aperture of elongated contour with its long dimension perpendicular to the electric vector of oscillations transmitted in said wave guide, the width of said aperture 'being sufficiently small at least for a part-of its length to permit the occurrence of an electric breakdown discharge across said aperture when oscillations in said pipe exceed a predetermined amplitude, said aperture being dimensioned and shaped so that said apertured barrier resonates at a frequency substantially equal to that of oscillations transmitted through said wave guide system, means associated with said barrier to maintain a partial vacuum in the space dened by said aperture and means located within said last-mentioned means for constantly maintaining ionization in the immediate neighborhood of said aperture.

d. In a wave guide system, a protective device comprising a conducting barrier of a shape which corresponds to the internal dimensions of said wave guide, said barrier being positioned transversely of said guide and having an elongated aperture adapted to resonate at a predetermined frequency, said aperture being suiiiciently small for a part of its length that an electrical breakdown discharge will occur across said aperture at a predetermined potential, means associated with said barrier and said aperture to maintain a partial vacuum in the space deiined by said aperture, and means for maintaining a region surrounding said aperture in an ionized state, said last-mentioned means comprising an opening in said conducting barrier communicating with said aperture, an electrode extending through said opening to said aperture, said electrode being insulated from said conducting barrier, and means for applying a voltage to said electrode.

'7. In a wave guide energy transmission system, a protective device which includes a conducting barrier positioned transversely of said guide and being formed with a centrally disposed elongated aperture, said aperture being dimensioned for resonance at the frequency of oscillations transmitted by said system, and being further dimensioned for incurring electrical discharge thereacross when the oscillations in said system exceed a predetermined energy level.

8. In a wave guide system, a protective device which includes a conducting barrier positioned transversely of the direction of propagation of electromagnetic energy in said wave guide and having an aperture located in said barrier, said aperture being so dimensioned in a direction parallel to the direction of the electric field of osl14 cillations insaid guide that a breakdown will oc-a cur across said aperture in said direction when the amplitude of said oscillations exceeds a predetermined value, said aperture being further dimensioned whereby said aperture is resonant at the frequency of said oscillations.

9. In a wave guide system, a protective device which includes a conducting barrier positioned transversely of the direction of propagation of electromagnetic energy in said wave guide and having an elongated aperture located in said barrier, said aperture being so dimensioned, at least in part, in a direction parallel to the direcf tion of the electric eld of oscillations in said guide that an electrical breakdown will occur across said aperture in said direction when the amplitude of said oscillations exceeds a predetermined value, the dimension of said aperture in a direction perpendicular to said breakdown direction being arranged whereby said aperture is resonant at the frequency of said oscillations.

10. Apparatus in accordance with claim 9, wherein said aperture includes enlarged openings at the ends thereof.

11. In a wave guide system having a junction of ilrst and second wave guides, a protective device which includes a conducting barrier located at said junction and positioned transversely of the direction of propagation of waves through said second wave guide, said barrier having a tuned aperture therein, said aperture being so dimensioned that it is resonant at the frequency of oscillations transmitted by said system and that a discharge will occur across said aperture when the oscillations in said system exceed a predetermined amplitude.

12. Apparatus in accordance with claim 11, wherein said tuned aperture comprises within said barrier a centrally disposed elongated slit oriented perpendicularly to the direction of the electric field of oscillations in said second wave guide.

13. Apparatus in accordance with claim 11, wherein said tuned aperture comprises within said barrier a centrally disposed elongated slit having enlarged openings at the ends thereof, said slit being oriented perpendicularly to the direction of the electric field of oscillations in said second Wave guide.

14. In a wave guide system having rst and second Wave guides wherein said second wave guide forms a junction with said rst wave guide at a wall of said rst wave guide, a protective device comprising a tuned aperture in the said Wall of said rst wave guide being oriented centrally of the area of said first wave guide enclosed by the end of said second wave guide at said junction.

15. Apparatus in accordance with claim 14, wherein said tuned aperture comprises an elongated slit oriented perpendicularly to the direction of the electric iield of oscillations in said second wave guide.

16. Apparatus in accordance with claim 14, wherein said tuned aperture comprises an elongated slit having enlarged openings at the ends thereof, said slit being oriented perpendicularly to the direction of the electric eld of oscillations in said second wave guide.

1'7. In a wave guide system having rst and second rectangular wave guides wherein said second wave guide forms a junction with said first Wave guide along a broad wall of said rst wave guide, a protective device comprising a tuned aperture in the said broad Wall of said rst wave guide, said aperture being oriented centrally of 15 the` area of" the broad wall offsaid first-wave guidel enclosed? byf the end? oisa-i'd second. wavev guide: atsaid junction.

1B'. Apparatus in accordance'y with claim 1.7; wherein'. said tuned aperture, comprises' an. elongated slit orientedV perpendicularly ofthe. direction off the electric. field of. oscillations; in. said second wave guide.

19. Apparatus in: accordance: with. claim: 1"1,l wherein saidl tunedy aperturey comprises an'. elongated-slit having enlarged openings at: thee. ends thereof.. saidl slit being' oriented. perpen'dicularly of 'the' directionlof'the eie'ctric'. field. of'oscillations insaidse'cond waveguide.

20.". Apparatus int accordance'. with'. claim'. 7, which'. includes;` in addition, means associated with. said barrier for maintainingl a partial. vac.- uum in: the space defined by said aperture for promoting. ionization and. electrical breakdown therein.

21'.. Apparatus in accordance with claim 7, which includes in'` additiorr an envelope secured to' said' barrier' and' enclosing said aperture` for maintaining a partial vacuum in thel space defined` by said: aperture to promote ionizationv and electricalibreakdown therein. 1

22.'. Apparatus' in accordanc'ewith claim '7, including means` associatedv with: saidl aperture for establishing: in the immediate vicinityy thereof a regionof'chargedielectric'particles.

23; Apparatus' in accordance withA claim 7, inoludingy means associatedy with said barrier an'd Said' aperture.' to'maint'ain. a partial,` vacuum in the' space. definedA by said' aperture,y and' means associated Withi said' aperture for' establishingin the immediate vicinity thereof! ay region of' chargedL electric particles.`

24'. Apparatus in' accordance. with claimffL-i'ncluding means for'ma'intaining. a-partialT vacuum in the space donned by said apertureandmeans forestablishinga region of charged electric par'- ticles in the immediate neighborhoodv of said aperture, said lasti-ment'ione'dv means` comprising an electrode extending into said aperture, and means for applying. a voltageto said electrode'.

25. In. combination, a dielectric waveguideot theh'oll'ow-pipe type, exciting meansfor estab'- lishing electromagneticA waves` within. said. guide,y a. metallic wall assooiatedwith said. guide and lying; in. a'. plane substantially transverse` to the direction of Wai/'e'v propagation' therethrough'- andv provided with anaperture having; anappreciabl dimension perpendicular to: the electric component of the waves propagated. through saidguidev and tuned tothe frequency ofthe exciting'mea'ns. and means for enclosing said; aperture in. an atmosphere at low pressure comprising a pair of bulbous vitreous members mounted on opposite sides of said' wall and sealed thereto.

ANDREW LONGACRE..

References Ci'tedv in the file of this patent- UNITED STATES PATENTS' Number Name Date 1,271,794: Stevenson July 9', 1918 2,396,044' Fox Mar. 51946.A 2,407,068' Fiske et al Sept. 3, 1946 2,407,069v Fiske Sept. 3,1946 2,413,963 Fiske et al'. Jan. 7, 1947 2,416,168 Fiske Feb. 181947 2,432g`093 Fox Dec` 9, 1947 

